In recent years, electric double layer capacitors capable of charge-discharge at high current have come to be considered promising as storage devices with high charge-discharge frequency, such as auxiliary power supplies for electric vehicles, auxiliary power supplies for solar cells and auxiliary power supplies for wind power generation. Consequently, electric double layer capacitors with high energy density, capable of rapid charge-discharge and exhibiting excellent durability, are in demand.
Electric double layer capacitors have a structure wherein a pair of polarizing electrode layers are situated facing each other across a separator as a positive electrode and negative electrode. Each polarizing electrode layer is impregnated with an aqueous electrolyte solution or nonaqueous electrolyte solution, and each polarizing electrode layer is connected to its respective collector. The aqueous electrolyte solution can raise the electrostatic capacity density and allows the resistance value to be lowered, but since the applied voltage must be no greater than the voltage at which water undergoes electrolysis, a nonaqueous electrolyte solution is used to increase the energy density.
Methods for producing polarizing electrodes include primarily sheet forming (rolling) methods and coating methods. Coating methods allow formation of thin electrodes by a relatively simple procedure, but pressing is carried out after coating onto a collector such as aluminum foil, and therefore the density of the electrodes cannot be increased. Sheet forming methods, on the other hand, accomplish connection with a collector such as aluminum foil after forming a sheet electrode that has been formed to a higher density than by coating methods, so that electrodes with high density can be easily obtained, and therefore such methods are most commonly used as connecting methods.
An example of an electrode for an electric double layer capacitor obtained by a sheet forming method is described in Japanese Unexamined Patent Application Publication No. 2005-136401, which proposes an electrode for an electric double layer capacitor having a polarizing porous sheet, comprising a carbonaceous electric double layer-forming material (active carbon or the like), a carbon material to ensure conductivity, and a binder-containing structural material, integrated on at least one side of a collector through a conductive interlayer, wherein the conductive interlayer comprises synthetic rubber and two or more carbon materials with different particle sizes. With this type of electrode for an electric double layer capacitor, it is possible to achieve low internal resistance, high electric capacity and high power density.
Japanese Unexamined Patent Application Publication No. 2002-353074 describes the use of a specific poly-N-vinylacetamide as a binder in an electrode paste for an electric double layer capacitor. However, Japanese Unexamined Patent Application Publication No. 2002-353074 neither mentions nor suggests the use of a poly-N-vinylacetamide-containing binder as a conductive adhesive for connection between a polarizing electrode layer and a sheet-like collector.